This invention relates to the area of power amplifier control circuits and more specifically to the area of power amplifier integrated control circuits for use in wireless applications.
Wireless connectivity between electronic devices is gaining popularity as wireless enabling technologies, such as Bluetooth (trademark) and IEEE 802.11b are integrated into an increasing amount of devices. Transceivers that comply with the Bluetooth (trademark) wireless specification operate in an unlicensed, 2.4 GHz radio spectrum thus ensuring communication compatibility worldwide. These transceivers use a spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz intervals and therefore provides a high degree of interference immunity while communicating using wireless technology.
In order for wireless devices to communicate, transmitters provide a communication signal to a receiver. Within these wireless devices, such as those using Bluetooth(trademark), there are power amplifiers for amplifying a signal to a desired power level before they are broadcast through a transmitter antenna of the wireless transceiver. Power amplifiers are used for amplifying low intensity, low amplitude electrical signals in order to produce a higher power and higher amplitude amplified output signal. Depending on the amount of gain provided by the amplifier, the amplifiers usually dominate power consumption of the transceiver.
Power amplifier performance is judged under realistic operating conditions in terms of Adjacent Channel Power Ratio (ACPR) measurement. Prior to taking an ACPR measurement, a signal residing in a desired frequency channel is modulated using a digital modulation scheme, such as that set forth in adherence to the IEEE 802.11b modulation standard and output power in an adjacent channel, with respect to the desired frequency channel, is measured. The ACPR is a ratio of electrical power in a desired frequency channel compared to that in another adjacent channel, thus giving an indication of frequency spreading of the modulated input channel. If the ACPR is high then there is no energy in the adjacent channel. If spectral re-growth occursxe2x80x94where electrical power is injected in frequency bands adjacent to the modulated input channelxe2x80x94then the ACPR is reduced.
When switching power amplifiers (PAs) on and off the switching causes xe2x80x9cspectral splatter.xe2x80x9d Spectral splatter is the generation of energy in adjacent channels. This spectral splatter is harmful especially for radio communication equipment that uses a narrow channel bandwidth. Skirts showing up in the adjacent channel as a result of modulation may affect sensitivity of radio reception. One way to reduce spectrum re-growth is to have gradual rising and falling switching edges, thereby eliminating high frequencies that are associated with sharp modulation transition edges.
Delay circuits, which can be used for providing gradual rising and falling switching edges, using resistor and capacitor networks and are known in the prior art. For instance: in U.S. Pat. No. 4,983,931, a resistor divider network is used to charge and discharge a capacitor. U.S. Pat. No. 5,108,133 provides a ramp circuit used for providing an EEPROM programming signal. U.S. Pat. No. 5,523,645 provides a circuit used for regulating a charge time of an output node of an amplifier on start up. U.S. Pat. No. 5,619,115 uses a current limiting circuit to charge and discharge a capacitor for purposes of charge integration. U.S. Pat. No. 5,825,218 discloses a voltage ramp generator used for discharging of a capacitor in two stages, where a discharging current source discharges the capacitor from a maximum voltage the capacitor reaches down to a low voltage above ground potential, where another discharging current source discharges the capacitor from the low voltage reached by the first source to ground. U.S. Pat. No. 6,369,626 uses a low pass filter circuit coupled to a capacitor for use in a delay lock loop circuit. Unfortunately these circuits are typically not geared towards amplifiers and more particularly feature longer rise times and fall times and would therefore not be useable for wireless applications. U.S. Pat. No. 6,169,886 a power amplifier output power level is controlled using a ramp circuit for use in amplifying an input signal of varying input power levels. However, the ramp used within this circuit is more applicable to signals having varying input power levels and not for amplitude modulating of the signal.
The longer the rise time or the fall time of a power amplifier the larger the reduction in spectral re-growth, however it is known to those of skill in the art that longer rise times and fall times are not conducive to fast data transmission rates. Some applications like Bluetooth (trademark) require rise times and fall times in the order of 2xcx9c4xcexcs, while other applications need shorter rise times and fall times.
It is therefore an object of the invention to provide a ramping circuit for use in amplitude modulation of a power amplifier in wireless applications in order to reduce spectrum re-growth occurring in an adjacent wireless channel as a result of amplitude modulation.
In accordance with the present invention there is provided a circuit for controlling a high frequency power amplifier for amplifying a signal for wireless transmission comprising: an output port, a capacitor electrically coupled to the output port, a current source-sink coupled with the capacitor for providing current thereto and sinking current therefrom, the current source operable in a first state mode to charge the capacitor and operable in a second state mode to discharge the capacitor, wherein, in use, a transition between said the first modestate and the said second state mode results in a delayed transition of an output signal at the output port between first and second output signal levels having a delay transition time of the delayed transition being other than substantially related to an RC time constant of the circuit.
In accordance with an embodiment, the circuit is absent a resistor in series with the capacitor for forming an RC circuit.
In accordance with an embodiment, the circuit is for use in a BlueTooth(trademark) wireless transmitter.
In accordance with another embodiment of the invention there is provided a circuit for controlling a high frequency power amplifier for amplifying a signal for wireless transmission comprising: an input port for receiving a first input signal for amplification thereof; an output port; a control port for receiving a control signal; a capacitor electrically coupled to the control port for providing a control signal thereto in dependence upon a charge on the capacitor; a source current mirror electrically coupled with the capacitor and a sink current mirror electrically coupled with the capacitor, the source and sink current mirrors for providing current to the capacitor and sinking current therefrom, the current mirrors operable in a first mode to charge the capacitor and operable in a second mode to discharge the capacitor; and, a power amplifier circuit coupled to the first input port for receiving the first input signal and for amplifying the first input signal in dependence upon the control signal, the amplified first input signal forming the output signal and provided at the output port; wherein, in use, a transition between said the first mode and the second mode results in a delayed transition of the control signal at the control port between first and second control signal levels, with a transition time the delayed transition being other than substantially related to an RC time constant of the circuit.
In accordance with another aspect of the invention there is provided a method of modulating a power amplifier output signal level comprising the steps of: providing a capacitor; receiving a control signal having one of a first level and a second level; initiating a charging of said capacitor at a linear rate when said control signal changes from the second level to the first level; varying the power amplifier output signal level from minimum level to a maximum level during a linear charging of said capacitor, where a time taken for said charging is a first transition time; initiating a discharge of said capacitor at the linear rate when said control signal changes from the first level to the second level; and, varying the power amplifier output signal level from the maximum level to the minimum level during a linear discharging of said capacitor, where a time taken for said charging is a second transition time.